The output power of an electric machine is critically dependant on accurate synchronisation of phase energisation and rotor position. The electric machine therefore requires mechanism for accurately determining the position of the rotor. A relatively cheap solution is a Hall-effect sensor.
Following manufacture and assembly of the electric machine, there is a tolerance in the position of the sensor relative to the stator and rotor. This tolerance in position results in an offset in the electrical angle of the signal output by the sensor. As a result, phase energisation and rotor position are not perfectly synchronised and thus the power and efficiency of the motor are reduced.
For many motors, the tolerance in the position of the sensor is not a problem. This may be because the size of the electric machine is relatively large and thus the tolerance in the position of the sensor is small in comparison to the circumference of the rotor. Consequently, any positional tolerance in the sensor results in a negligible offset in the electrical angle of the sensor signal. Alternatively, the output power and/or the efficiency of the electric machine are not critical and thus the power losses that arise from the positional tolerance of the sensor are deemed acceptable. However, for electric machines that are relatively small and/or where relatively high efficiency is required, tolerances in the position of the sensor present a significant problem.